[xwiki-users] admin user login problem

Hi all, I have a Admin user account with my wiki setup, starting some time ago, when I login, I type Admin, then type admin password, the Admin user name field automatically turns into admin, which has lower case "a", that means different user account, which is not Administrator account, I have no way to login as Administrator, I have to use firebug debugger to open the underline html field and put Admin as the value to login. Any way to fix this? Thanks Dave